One of three things:
1 Worn out Tie Rods
2 Worn out steering gear box
3 Worn out linkage in steering shaft
It's probably #1 or #2. So check your tie rods. If they are loose: problem found. If they are tight then move on to the steering gear box. There is an adjustment screw on the gear box. Tighten that up a bit and see if that helps. If so, tightening the adjustment screw is just a short term fix, you need to replace the gear box. If both of those check out it's probably the u-joint in your steering shaft.